1. Field of the Invention:
The present invention relates to a secondary air supply system for supplying secondary air into the exhaust system of internal combustion engines for the purpose of purifying exhaust gases.
2. Description of the Prior Art:
It is known to employ a secondary air supply system as a counter measure for purifying exhaust gases from internal combustion engines so that harmful uncombusted components such as HC and CO contained in the exhaust gases are further combusted in the exhaust system under a supply of secondary air. A secondary air supply system such as this generally comprises a rotary air pump such as a vane pump additionally mounted to a conventional internal combustion engine to inject compressed air into the exhaust system of the engine by way of an air injection manifold. As an alternative, it has also been proposed to utilize exhaust gas pulsation in the exhaust system for injecting secondary air into the exhaust system by employing a system comprising a passage means connected at one end thereof to the exhaust pipe and opened at the other end thereof to the atmosphere and a check valve provided in the midst of said passage means, said check valve being adapted to allow air to flow only toward the exhaust pipe so as to effect pumping action to feed air into the exhaust pipe. The system employing the air pump has an advantage that it is capable of supplying any required quantity of secondary air into the exhaust system if a pump of a suitable capacity is employed the system employing the check valve to utilize exhaust gas pulsation to produce a pumping action has an advantage that a secondary air supply system is obtained by a simple structure such that a check valve is provided in the midst of a secondary air supply passage. Obviously, therefore, the latter system depending upon the exhaust gas pulsation is cheaper than the system employing an air pump. However, the latter system has a limitation with respect to the capacity thereof and is unable to supply a large amount of air sufficient to accomplish the required purification of exhaust gases, particularly when used with a large piston displacement.
On the other hand, it is favorable that the amount of secondary air injected into the exhaust system is maintained at a constant ratio with respect to the amount of engine intake air. This, of course, means that if the rotational speed of the engine increases, a larger amount of secondary air is to be supplied into the exhaust system. However, when the supply of secondary air depends upon the exhaust gas pulsation, the amount of secondary air actually injected into the exhaust system decreases as the rotational speed of the engine increases because the amplitude of the exhaust gas pulsation decreases in accordance with an increase in the rotational speed of the engine. Consequently, the performance of secondary air supply in this system is contrary to the actual requirement for secondary air.
Now, in internal combustion engines there also exists another pulsating phenomenon in the air flow in the intake system thereof and this intake air flow pulsation has a characteristic that the amplitude of the pulsation increases in accordance with decrease of the intake vacuum by contrast to the characteristic of the exhaust gas purification.